The Dragon spacecraft is being certified for manned spaceflight. If it's to replace Soyuz, though, it will have to serve as an escape pod for the crew. The current delivery missions are proving its unmanned variant as a reliable spacecraft capable of delivering the payload and surviving reentry. It seems not much is missing until its manned variant is ready and capable of bringing astronauts to ISS.
And then, if the flight was to be similar to that of Soyuz, it would dock to ISS and spend about seven months there (possibly more if Russians don't retire Soyuz from own flights), exposed to vacuum, UV, cosmic radiation, violent temperature changes, solar wind, possibly coronal mass ejections, its own corrosive propellants, and the fungus that tries to find any survivable niches on the ISS. Then it will have to be boarded, sealed, undocked, deorbited using own engines and 7-months-old propellant, then survive the reentry, deploy the parachutes and splash down safely.
Currently, the longest Dragon mission, SpX-9, lasted 36 days. How is the manned variant (to be?) tested and prepared for the long haul? Will there be a first unmanned verification flight of long duration, or will it all depend on simulations, ground tests and calculations of its long-term endurance?